Islam says killing another Muslim intentionally puts your ass in hell for life and as you know there is no other crime in Islam for which a Muslim spends time in hell eternally aside from shirk. Even when it comes to non Muslims, you can't randomly kill non Muslims. The prophets ways are clear to those of us who have learned and we can tell who claim to follow him and who actually follows him.
But when they're claiming you're the one that's not following the prophet, you have a problem.